If your partner is likely to devour you after sex, snapping off your genitals inside her might seem a reasonable reproductive strategy. This game plan is used by males of the orb-web spider Nephilengys malabarensis and, it turns out, continues to work in their favour, regardless of whether they survive the encounter.

Thus begins my new piece for Nature News. Honestly, I can’t believe they let me keep the lede. Here’s more:

Daiqin Li at the National University of Singapore and his colleagues studied the species and found that after the male breaks away his severed organ continues to pump sperm into the female. This allows him to fertilize her remotely, while denying entry to other males. Even though the male cannot regrow his genitals and so renders himself sterile, he increases the odds that he will father the offspring of his one and only mate.

Male spiders deliver their sperm through a pair of structures known as palps, which are found on the sides of their heads. By serving sexual encounters between 25 pairs of virgin N. malabarensis, Li’s group found that every coupling ended with damage to the male’s palp. In 12% of cases it was partially severed; in the rest it snapped off completely.

Li thinks that this bizarre strategy, found in only two spider families so far, evolved to counter the female’s penchant for cannibalism. “The females are very aggressive and 75% of them kill the males during sex,” he explains. “The duration of copulation is also very short, and the females initiate the break-off.”

By dissecting the mated spiders, Li and his co-workers found that the palp has dispensed only about one-third of its sperm by the time the female pushes the male off. But it continues to transfer sperm after it breaks off, and does so at a faster rate.

And head over there for the rest of the story.

(In the picture at the top, the bigger female devours the smaller male while his palp clings on to her underside (in the red box).)

We don’t like blurry vision, and we go out of our way to correct it with glasses and contact lenses. But some animals aren’t so fussy. The jumping spider not only tolerates blurry images, it deliberately produces them.

Jumping spiders, as their name suggests, leap onto their prey from afar. They judge their jumps using the two huge (and rather beautiful) eyes on the front of their faces. And to gauge how far away their targets are, they use special retinas that produce sharp images and out-of-focus ones at the same time.

Other animals have many different ways of judging depth, but none of them apply to jumping spiders. Humans mostly rely on our two eyes. Each gets a slightly different view of the world and our brain uses these differences to triangulate the distance to objects in front of us. But this ‘binocular vision’ only works if the two eyes see overlapping parts of the world. Those of jumping spiders do not.

Chameleons can judge distance by sensing how much they have to focus their eyes to bring an object into sharp relief.  But jumping spiders have no way of actively focusing their eyes. Finally, some insects judge distance by shaking their heads from side to side, which makes nearby objects move further across their field of view than far ones. But jumping spiders can accurately pounce onto their prey without moving their heads.

Without any of these three methods, how could they possibly gauge their precise killing pounces with any sort of accuracy? Takashi Nagata from Osaka City University has the answer.

Each of the front eyes has a unique staircase-shaped retina, with four layers of light-sensitive cells lying one over the other. By contast, our retinas only have one such layer. Scientists have known about the staircase retinas since the 1980s, but Nagata has finally shown exactly what they do.  He found that the top two layers are most sensitive to ultraviolet light. The two on the bottom have a penchant for green.

And that’s a bit odd. The way the layers are stacked means that green light only ever focuses sharply on the bottom one (layer 1). Blue light focuses on the one above it (layer 2), but those cells aren’t sensitive to blue. Instead, they see the world in fuzzy out-of-focus green.

Nagata thinks that this fuzzy vision isn’t a bug; it’s a feature. The amount of blur depends on an object’s distance from the spider’s eye. The closer it is, the more out of focus it is on the second retina. Meanwhile the first retina always gets a sharp image. By comparing the images on both layers, the spider can gauge depth with a single unmoving eye.

To test this idea, Nagata placed Adanson’s house jumpers in a special arena where they had to leap at prey. If the arena was flooded with green light, the spiders made accurate jumps. If Nagata used red light of equal brightness, they fell short of the mark. Nagata even created a mathematical model for the spider’s eye to predict how far it would miss its jump under different wavelengths of light. The model’s predictions matched the animal’s actual behaviour.

Humans actually do something similar. We can use the blurry nature of background images to get a sense of distance, even if all other cues are removed. Indeed, photographers often use blurry backgrounds to create a greater sense of depth. But this is just one of the tricks we use to judge depth, and perhaps a minor one. For the jumping spider, it seems to be the only trick in the playbook.

Reference: Nagata, Koyanagi, Tsukamoto, Saeki, Isono, Shichida, Tokunaga, Kinoshita, Arikawa & Terakita. 2011. Depth Perception from Image Defocus in a Jumping Spider. Science http://dx.doi.org/10.1126/science.1211667

Photo by Alex Wild

The eyes have it – a tour through the stunning world of animal eyes

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The two apes above might look very similar to the untrained eye, but they belong to two very different species. The one on the right is a bonobo; the one on the left is a chimpanzee. They are very closely related but the bonobo is slimmer, with a smaller skull, shorter canines and tufts of lighter fur. There are psychological differences too. Bonobos spend more time having sex, and playing with one another. They’re less sensitive to stress. They’re more sensitive to social cues. And they are far less aggressive than chimps.

Many years back, a young researcher called Brian Hare was listening to the Harvard anthropologist Richard Wrangham expound on this bizarre constellation of traits. “He was talking about how bonobos are an evolutionary puzzle,” recalls Hare. “They have all these weird traits relative to chimps and we have no idea how to explain them.”

But Hare had an idea. “I said, ‘Oh that’s like the silver foxes!’ Richard turned around and said, ‘What silver foxes?’”

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This is an updated version of an old piece, edited to include new information. Science progresses by adding new data to an ever-growing picture. Why should science writing be different?

Right from its entrance, Disneyland is designed to cast an illusion upon its visitors. The first area –  Main Street – seems to stretch for miles towards the towering castle in the distance. All of this relies on visual trickery. The castle’s upper bricks and the upper levels of Main Street’s buildings are much smaller than their ground-level counterparts, making everything seem taller. The buildings are also angled towards the castle, which makes Main Street seem longer, building the anticipation of guests.

These techniques are examples of forced perspective, a trick of the eye that makes objects seem bigger or smaller, further or closer than they actually are. These illusions were used by classical architects to make their buildings seem grander, by filmmakers to make humans look like hobbits, and by photographers to create amusing shots. But humans aren’t the only animals to use forced perspective. In the forests of Australia, the male great bowerbird uses the same effect to woo his mate.

Bowerbirds are relatives of crows and jays that live in Australian and New Guinea. There are 20 or so species. In most of them, the male attracts mates by building an intricate structure called a bower, which he decorates with specially chosen objects. Some species favour blue trinkets; others collect a  mishmash of flowers, fruits, insect shells and more. Surrounded by these knick-knacks, the artistic male performs an elaborate display. The females judge him on his skill as a performer, builder and decorator.

The great bowerbird’s taste for interior design seems quite Spartan compared to his relatives. He creates an avenue of sticks, around 60 centimetres long, leading up to a courtyard. The courts are decorated with gesso – a collection of gray and white objects including shells, bones and pebbles.

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To fans of cheesy pop music, the beat of someone else’s heart is a symbol of romantic connection. To a boa constrictor, those beats are simply a sign that it hasn’t finished killing yet.

A constricting snake like a boa or a python kills its prey by suffocation. It uses the momentum of its strike to throw coils around its victim’s body. Then, it squeezes. Every time the prey exhales, the snake squeezes a little more tightly. Soon, the victim can breathe no more.

We’ve known this for centuries but amazingly, no one has worked out how the snakes can tell when to stop constricting. Scott Boback from Dickinson College has the answer. Through its thick coils, a boa can sense the tiny heartbeats of its prey. When the heart stops, the snake starts to relax.

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A typical starfish has five-sided symmetry. With no clear head, the starfish can move in any direction, led by any one of its five arms. If you were feeling particularly cruel, you could fold one up in five different ways, so each half fitted exactly on top of the other. We humans, like many other animals, have only two-sided symmetry. We’re ‘bilateral’ – our right half mirrors our left, and we have an obvious head.

These two body plans might look radically different, but looks can be deceiving. Chengcheng Ji and Liang Wu from the China Agricultural University have found that starfish have hidden bilateral tendencies, which reveal themselves under times of stress.

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“God save thee, ocean sunfish
From the fiends that plague thee thus
Why look’st thou so? With thy large shoals,
Thou fed the albatross.”

- Samuel Taylor Coleridge, sort of.

Albatrosses are superb long-distance fliers that can scour vast tracts of ocean in search of food. But sometimes, food comes to them. In July 2010, Tazuko Abe from Hokkaido University found albatrosses cleaning a school of ocean sunfish, basking at the surface of the western Pacific Ocean.

The ocean sunfish is a truly bizarre animal. It looks like someone cut the head off a much bigger fish and strapped fins to it. It’s the largest of the bony fish*. The biggest one ever found was 2.7 metres in length and weighed 2.3 tonnes. The youngsters, of course, are much smaller. The ones that Abe saw on his research cruise were just 40 centimetres long. There were at least 57 of them, each turned on its side so its broad flank faced the water surface.

The basking shoals were attending a sort of sunfish spa. The fish were infested with parasites. Pennella, a long scarlet relative of shrimp and crabs, was embedded headfirst in the flesh beneath their fins, busily sucking their blood. But not for long – black-footed and Laysan albatrosses were attracted to the shoal and picked the Pennella off their bodies. In some cases, the sunfish seems to be courting the birds, following them around and swimming sideways next to them.

Ocean sunfish live throughout the oceans but they often spend time at the surface before diving to the depths. Some scientists think that they’re absorbing heat from the sun, but it’s possible that they could also be looking for a spot of personal hygiene.

These fish can play host to at least 50 species of parasites, and they often have considerable numbers on their large bodies. Many ocean animals rely on cleaner fish or cleaner shrimp to rid them of parasites. It’s possible that albatrosses might fulfil the same role for ocean sunfish.

Of course, the association might have been a one-off. However, there are other reports of seabirds such as shearwaters and albatrosses flocking around schools of basking sunfish. This instance stands out only because Abe has photographic evidence that they were actually parasites. As he rightly points out, such events would be difficult to spot among the vastness of the open ocean.

* Fish have skeletons that are either made of cartilage, as in sharks and rays, or bone, as in all the others.

Reference: Abe, Sekiguchi, Onishi, Muramatsu & Kamito. 2011. Observations on a school of ocean sunfish and evidence for a symbiotic cleaning association with albatrosses. Marine Biology http://dx.doi.org/10.1007/s00227-011-1873-6

In December of 2011, Fred Kraus from the Bishop Museum in Hawaii announced that he had discovered the world’s smallest frogs. The two coin-sized species were just 8.1 to 9.3 millimetres long. But these miniscule amphibians now share a different record – they were the world’s smallest frogs for the shortest amount of time.

Less than a month after Kraus’s announcement, Eric Rittmeyer and Christopher Austin from Louisiana University have found an even smaller frog, just 7 to 8 millimetres long. It’s dwarfed by a dime. It’s not just the world’s smallest frog, but the world’s smallest back-boned animal.

The new species, Paedophryne amauensis, is a close relative of the tiny pair from December – Paedophryne dekot and Paedophryne verrucosa). Extremely tiny frogs have evolved at least 11 times, but the Paedophryne group is unique in that all of its members are miniscule. They were first discovered in 2002, and six species have been discovered so far. All of them live in Papua New Guinea. Clearly, this corner of the world is a haven for the tinier side of life.

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Philcoxia couldn’t look more unassuming. It’s a small herb that lives in Brazil’s Campos Rupestres region, a sparse plateau of rocky outcrops and white sands. All you’d see of it are a handful of twigs sticking out from the grains, topped with small purple flowers and even smaller leaves. You wouldn’t think that it’s the type of plant that can kill animals.

To find Philcoxia’s grisly secret, Caio Pereira had to look underground. The plant biologist from Unicamp, Brazil, found that the plant traps and digests tiny worms with sticky underground leaves.

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One massive group of ants has a secret supersoldier programme that’s been locked away for 35 to 60 million years.

The Pheidole ants are an exceptionally diverse group with over 1,100 species. They’re also known as big-headed ants because their soldier caste has unusually large heads. Until now, we knew that a few of the Pheidole – just 8 out of 1,100 – can also produce supersoldiers, which are even larger than normal soldiers and have even more enormous heads. They use their outsized noggins to block their nest entrances against invading army ants.

Now, Ehab Abouheif has found that the supersoldiers are the result of a genetic programme that runs throughout the entire Pheidole dynasty. It’s likely that every single species in the vast group has the hidden ability to make this special caste. In fact, Abouheif managed to induce supersoldiers among species that don’t usually recruit them, with just a dab of hormone.

I wrote about this study for Nature, so head over there to read all the details. It’s a great evolutionary story.

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